Blockages in the flow of oil during production are common and are often due to the deposition of heavy organics from petroleum fluids. These heavy organics, such as paraffin and asphaltenes, exist in crude oil products in various quantities and forms a precipitate or crystallize into solids when temperature or pressure drops in the oil production process. These heavy solids stick to the walls of tubulars and to reservoir surfaces, resulting in blockage of flow. These solids can also pick up coal, limestone, sand or other fines, further thickening the material, and further contributing to blockage.
Heavy organics, in many instances, move from the well tubing to flow lines and to production separators, pumps, strainers and other fluid-handling equipment, creating further operational problems. In many cases, oil wells are completely shut down after being plugged by these heavy materials.
The principal difficulty in designing a program for control of paraffin is the uncertainty of the composition of the deposit. Well deposits can vary from a mushy consistency at the upper end of the deposit to hard, crystalline waxes at the bottom. The type of wax is a function of the carbon atoms in the molecule (carbon number), and the melting temperature increases with the carbon number. Generally, wax components in the carbon number range of C12 to C25 are classified as soft and those in the C25 to C50+ range as hard, and hard deposits are In pipeline deposits where temperature and pressure remain relatively constant, the wax will generally be of a uniform composition harder to remove with thermal treatments.
Wax composition and temperature are the primary factors determining where deposition will occur, but pressure can be of major importance in partially depleted reservoirs with low bottom hole temperatures. In addition to higher temperature maintaining the wax in solution, the light ends, i.e., propane, butanes and pentanes, increase the solubility of the wax in the reservoir hydrocarbons. As the reservoir pressure depletes, these condensate fractions enter the gas phase and waxes precipitate. In some shallow fields with low bottom hole temperatures, wax deposition may occur in the reservoirs, further complicating the control and removal treatments.
Current methods addressing the paraffin problem are as follows:
Mechanical Removal:
The use of wireline cutting tools and flowline scrapers is the oldest method of removing wax deposits, and costs roughly the same as thermal techniques. It is possible with some systems to perform scraping operations while the well is producing, allowing the operator to keep production levels high. However, it is important in both well bore and flowlines to schedule cleanings at frequent enough intervals.
Scraping techniques are well established, but they are not without disadvantages. Scraping can result in large chunks of scraped paraffin settling in flowlines or surface equipment causing blockage. Metal to metal contact also roughens the tubing wall thus encouraging deposition, bacterial growth, and corrosion.
Chemical Remediation and Prevention:
Chemical solvents, dispersants and wax crystal modifiers have often been used in conjunction with the mechanical and thermal techniques to enhance the effectiveness of the treatments.
Chemical solvents and dispersants may also be effective in removing deposits from wells and flowlines depending on the degree of blockage. Wax dispersants and solvents can be used in conjunction with mechanical and thermal techniques to improve efficacy in the removal of deposits. Regular batch treatments or continuous injection can keep wax deposition under control, but mechanical and thermal techniques may still be needed on a frequent basis.
Wax crystal modifiers are used to prevent or inhibit wax deposition. Where applicable, wax crystal modifiers can significantly reduce the need for remediation and lower operating costs.
However, while chemical treatments may help to manage paraffin issues in the well, chemicals do not suspend the paraffin indefinitely and can be damaging to the environment. Additionally, chemical treatments can partially dissolve or disperse paraffin, only to have it recrystallize further down the line in another area of the well. Chemical treatments are often not sufficient of themselves to address the paraffin problem.
Thermal removal: Next to scraping, removing paraffin in well bores and flow lines by melting the deposits has the longest history of continuous use in production operations. Melting the wax is usually by the injection of hot oil or hot water. This technique is simple, low cost and the results are immediate.
U.S. Pat. No. 2,704,979 for example, describes the use of nested tubes, the inner of which is connected to a pump, wherein hot oil is run down the inner tubing, out the ports, and up the annulus to melt paraffin deposits, which are then carried away with the heated medium. The flow path can also be reversed, sending hot medium down the annulus, and bring the hot medium plus melted heavies up the inner rods. Typically hot oil treatments are used that require a hot oil truck to bring hot oil to the field. Hot oil is run through the system, and produced with paraffin dissolved in oil. Problems occur as the hot oil cools and wax again congeals. If melted wax enters the formation, especially in wells less than 160° F., the congealed wax can cause permeability damage.
Although simple and at least partially effective, thermal treatments are not a panacea, particularly when insufficient amount of heated medium are provided and/or for insufficient time to thoroughly melt all heavy deposits, in particular the harder deposits with higher melting points.
When hot fluid is injected into a well, the heat of the fluids rapidly transfers to the cooler well bore equipment and waxes in the upper sections of the well. As pumping of the hot treating fluid continues the heated zone progresses down the well bore removing additional deposits. However, the treatment will not completely clean the well unless the treating fluid is hot enough and injected long enough to reach the melting point of the hard waxes at the lower end of the deposit.
Initially, only a thin layer of hard wax remains in the well when insufficient heat was applied. With continuing inadequate treatment, the thickness of the hard, crystalline wax layers increases, until eventually, flow is restricted and the heat treatments are no longer effective. This problem can be further complicated in wells with low bottom hole temperatures. The deposit can form over the producing interval and may extend into the reservoir. This can create skin damage that complicates well servicing procedures.
Hot oilers commonly draw the oil needed for hot oiling from the bottom of storage tanks nearest to the well. These tanks will usually contain higher quantities of wax due to prior hot oil treatments or wireline scraping. The tank bottoms may also contain solids such as iron sulfide, clay, sand and iron oxide. The potential for damaging the well goes up when using oil from the bottom of the tank.
There is another hidden cost in hot oiling that often is overlooked. Oil is lost during hot oiling due to its volatility. Hot oil trucks hold 60-75 barrels of oil and as much as 3-10 barrels can be lost during each hot oil job. Loss of light ends not only reduces crude volume, but also results in a higher concentration of wax components being pumped to the bottom of the well. After one considers all the risks, it may be more desirable to use hot water instead of hot oil.
Although U.S. Pat. No. 2,704,979, discussed above, states that hot water can be used in thermal removal methods, water based methods are not exemplified and no details are given. Thus, one can only assume that the same methods used for hot oil would be used for hot water, although in our experience, these are not suitable, or at least less suitable. For example, waxes are not soluble in water, and thus the methods will need to be modified to accommodate this reality.
In-situ heat generating techniques have been successful in certain situations. These are several patented processes that all basically do the same thing. They involve the careful placement of chemical solutions near the paraffin deposit that react when they come into contact with one another to generate large amounts of heat. Some of the processes also generate nitrogen that can also aid in the recovery of production. Some of the hazards associated with in-situ generating techniques are rapid pressurization, potential corrosion, and fire. Field operations should be aware of all of the associated hazards of using these techniques and they should only be performed by qualified personnel.
Paraffin removal by heating is a reasonably effective method for control of wax depositions when the limitations are recognized and treatments are designed and implemented to assure complete removal, but as with the other techniques, the limitations must be recognized, and it is easier to remove soft waxes than hard.
None of the above methods is thus completely satisfactory, and paraffin deposits continue to plague the industry. Much has been spent in research on methods for preventing or removing paraffin deposits, but no universal solution has ever been found and the paraffin problem is as insidious today as it ever was.
There remains further need in the art to develop methods of addressing the paraffin buildup in producing wells.